1. Technical Field
The invention relates to the field of inkjet printing. More specifically, the invention relates to a process for controlling the composition of an atmosphere exposed to a curable ink in a radiation curing print process.
2. Description of the Related Art
Inkjet printing involves producing a digital image on a substrate by propelling droplets of liquid material (ink) onto the substrate. Inkjet printing solutions can involve using base coats, electromagnetic radiation, curing, and inerting a print region with an inerting atmosphere.
Some printing solutions involve applying a base coat to a substrate before printing a desired image. For example, in order to print color images on non-white substrates, such as colored or transparent substrates, it is typically necessary to deposit a layer of white ink to serve as a backdrop for the color inks. Also, to print a multi-colored image on a black or colored substrate, the area of the substrate on which the image is to be printed is first pre-coated with a layer of white ink, and then the image is printed on top of the white pre-coat layer. The white background layer prevents the colors in the image from being distorted by the black or colored substrate
Additionally, when printing on a transparent substrate, the colored inks may be applied on the reverse side of the substrate, so that the image may be viewed through the front side of the substrate. Then, a layer of white ink is printed over the colored ink pattern in a post-coating step. The white “post coat” layer serves as a backdrop so that the colors of the image appear properly when viewed from the front side of the transparent substrate. In some cases, the transparent substrate is then laminated onto a second transparent substrate, such as a window, so that the color image is protected between the two transparent substrates.
The Applicants have developed methods and apparatus for printing a coating layer in co-pending United States Patent publication no. 20060158473, filed on Jan. 19, 2006, entitled Methods and apparatus for backlit and dual-sided imaging, which is incorporated herein in its entirety.
According to United States Patent publication no. 20060158473, an array of print heads arranged along a single print head axis is configured to print images and a coating layer on a substrate during a single printing step (i.e., without requiring separate pre-coat or post-coat processing). In particular, a print apparatus deposits a first image layer on a substrate, then deposits a coating layer over the first image layer, and then deposits a second image layer over the coating layer.
The coating layer may comprise a specialized printing fluid such as a substantially white ink. The substrate is oftentimes a substantially translucent or substantially clear material, such as glass or plastic media. Indeed, these printing techniques are useful for backlit imaging and dual-sided imaging.
Although basic base coating techniques have been previously developed, there is a need in the art for methods and systems for controlling the quality and characteristics of the base layer, wherein these characteristics affect the overlaid image. Currently, characteristics such as dot gain, interlayer adhesion and slip are controlled by using additives such as silicone based surfactants.
Additionally, an inert gas, such as nitrogen or carbon dioxide is commonly used in radiation curable processes to enhance cure speed, particularly surface cure by reducing oxygen that reduces cure speed as a result of competing triplet and radical quenching reactions.
Some printing solutions also involve light curing of inks. Known ink-curing techniques involve using a specific ink formulation and exposing it to energy from an electromagnetic radiation source. The goal in both conventional and inkjet printing is to enable cure with reduced dose and or power of actinic radiation. Curing of liquid chemical ink formulations has been an established practice for many years. In ultraviolet curing, a liquid chemical formulation comprising photoinitiators, monomers and oligomers, and possibly pigments and other additives is exposed to ultraviolet light, thereby converting the liquid chemical formulation into a solid state.
Curing ink involves directing photons, typically with wavelengths in the ultraviolet spectrum, onto an ink deposit. The photons interact with photoinitiators present within the ink, creating free radicals. The created free radicals initiate and propagate polymerization (cure) of the monomers and oligomers within the ink. This chain reaction results in the ink curing to a polymer solid. However, the presence of oxygen at the ink surface inhibits such a chain reaction from occurring within the ink. This is often referred to as oxygen inhibition.
In normal ultraviolet curing in an air environment, a high amount of ultraviolet energy and/or a high concentration of photoinitiator are needed to achieve full cure, compared to the ultraviolet power and photoinitiator concentration required in an oxygen free curing environment. Higher photoinitiator concentration may deleteriously affect the final film properties, and increase ink costs. Higher ultraviolet energy required to overcome oxygen inhibition increases power requirements and heat generated on the sample.
Common solutions for providing for less reactive curing include completely supplanting atmospheric oxygen with a less reactive gas such as nitrogen in the cure zone. For example, U.S. Pat. No. 6,126,095 to Matheson et al., entitled “Ultraviolet Curing Apparatus Using an Inert Atmosphere Chamber” teaches a curing apparatus comprising a curing chamber for accommodating a controlled atmosphere. The curing chamber includes inlets and nozzle assemblies for supplying less reactive gas into the chamber and maintaining a less reactive atmosphere therein.
The prior art involves specialized and expensive approaches to providing reduced oxygen curing conditions, but fall short of achieving feasibility for common inkjet printing systems. For example, curing chambers demand a large footprint and are typically expensive to obtain, operate, and maintain. Additionally, large curing chambers have unacceptable levels of power consumption and heat production, requiring the use of heat sinks and other cooling systems.
According to the current state of the art, while adding a surfactant to an undercoat such as a clear or white, enables sufficient spread and a smooth surface, the adhesion and print quality of the subsequent printed layer may be negatively impacted. This is particularly pertinent to inkjet printing where drops must spontaneously spread to cover the surface and there is no contact pressure to enhance spread that is found in many conventional printing processes. For ink jet printing, some of the above mentioned current practices, such as the use of particulate matting agents, are not accessible. This is because the size of the particulate, in order to be effective, exceeds the size that the print-head can accommodate.
Additionally, the majority of current ink-curing solutions utilize high pressure arc lamps for curing. However, there are several drawbacks to these techniques.
First, typical light-curing systems that use arc lamps possess a very large physical footprint. In the case of a system for base coat printing followed by a top coat, a first printer having a UV curing station sets down and cures the base coat while an additional printer is required to set down the top coat. It would be highly beneficial to reduce the physical size of printers with light-curing stations. Likewise, it would be highly beneficial to eliminate the need for two printers in a two-step printing process.
Also, known current light-curing systems that use high pressure arc lamps produce a very high level of heat. This high level of heat prevents a traditional curing lamp from being placed in-line with other printing processes. Accordingly, heat sinks are required to remove excess heat. Likewise, traditional light-curing printing techniques release ozone which must be evacuated or otherwise removed.
Therefore, there is a need in the art for a solution that provides adequate curing, without requiring a large footprint, without requiring large amounts of power, and without producing unacceptable levels of heat while at the same time maintaining acceptable levels of print quality and interlayer adhesion.